A known cellular wireless communication system is a Code Division Multiple Access (CDMA) system. The CDMA wireless phone system allows multiple cellular phone users to share the same frequency spectrum, and uses a generated Pseudo Noise (PN) code with a different and essentially orthogonal instance of the PN code assigned to each mobile unit within a cell. A base transceiver station receiver in a CDMA station correlates the received signal from a mobile station with the desired PN code, extracting the transmitted digital signal with a sufficient signal-to-noise ratio to achieve a satisfactory data error rate.
The general structure and operation of cellular wireless communication systems are generally known. A cellular network infrastructure typically includes a plurality of base transceiver stations that each services wireless communications for one or more cellular mobile stations within a respective cell. Typically, each base transceiver station supports a plurality of sectors within its serviced cell. A base station controller typically services a plurality of base transceiver stations and coordinates operations within the cells serviced by the base transceiver stations. A mobile switching center services a plurality of base station controllers and couples to the Public Switched Telephone Network (PSTN). Typically, the base station controllers or the mobile switching center couple to the Internet to service packetized communications.
Cellular wireless infrastructures typically support one or more wireless protocol standards. These wireless protocol standards include the CDMA protocol standards such as IS-95A, IS-95B, 1x-RTT, 1xEV-DO, 1xEV-DV, UMTS, and other CDMA type protocols. Alternately, the wireless protocol standard may service a Time Division Multiple Access (TDMA) standard such as the GSM standard, the North American TDMA standard, or other TDMA standards. The cellular mobile stations operating in the service area communicate with the base transceiver stations using such supported wireless protocol standards.
As is known, transmissions from a base transceiver station to a cellular mobile station thereof are called forward link transmissions. Likewise, transmissions from cellular mobile stations to base transceiver stations are called reverse link transmissions. The cellular network infrastructure coordinates and manages both the forward link and reverse link transmissions. Due to mobility of the cellular mobile stations, the forward link transmission power and the reverse link transmission power are controlled. In CDMA systems, for example, the reverse link transmission power and the forward link transmission power must be closely controlled for each cellular mobile station. Existing CDMA wireless protocol standards provide strict guidelines for a closed loop power control. With these standardized operations, a servicing base transceiver station controls the reverse link transmission power by sending (as necessary) power control bits on the forward link to each serviced cellular mobile station. These power control bits are typically contained in the power control sub-channel. For each power control bit, the cellular mobile station either increases its reverse link transmission power or decreases its reverse link transmission power, depending upon the value of the power control bit.
Prior cellular systems are predisposed to lose the reverse link. In order to reduce interference within a service sector or cell, prior art servicing base transceiver stations direct their service cellular mobile stations to transmit at a minimum acceptable reverse link transmission power level via use of the power control bits accordingly. Thus, reverse link transmissions typically arrive with minimally sufficient power at the servicing base transceiver station. With such prior systems, when the reverse link is lost, the base transceiver station (or the servicing base station controller) typically considers the mobile station as out of track and tries to keep forward link transmissions at the current power level, which may be too low for the mobile station to decode. Thus, many calls are dropped due to these prior power control operations.
Problems with prior power control operations are often caused by errors on the power control sub-channel, where a power-up bit transmitted by the base transceiver station may be incorrectly demodulated by the mobile station as a power-down bit. These “presumed” power-down bits cause a receiving cellular mobile station to reduce its reverse link transmission power when it should increase its reverse link transmission power. When a cellular mobile station is in soft handoff between base transceiver stations or softer handoff between sectors of the base transceiver station in a CDMA system, the cellular mobile station receives multiple power control bits from the multiple currently serving sectors. However, the cellular mobile station is disposed to more strongly consider a power-down power control bit than power-up power control bit. Thus, the cellular mobile station in such case is predisposed to reduce its reverse link transmission power which results frequently in loss of the reverse link. Thus, in many such prior operations, calls are dropped because of the failure to properly control the reverse link transmission power of the cellular mobile station.
It is, thus, necessary for controlling the reverse link transmission power of the cellular mobile station to correctly demodulate a power-up bit transmitted by the base transceiver station.
In CDMA 1xRTT and WCDMA (UMTS), a closed loop fast power control is introduced on the forward traffic channel to combat fast fading environments. This mechanism contributes to the substantial increase in the forward link air-interface capacity in comparison to IS-95. The forward link power control commands are decoded from the reverse power control sub-channel embedded on the reverse pilot channel. In one currently commercialized modem, for example, when there is at least one locked finger, the decoded binary power control bit is used for adjusting the forward link traffic channel gain either up (binary 0) or down (binary 1). If all the fingers are out of lock, no adjustment on the forward link traffic channel gain will be performed.
A problem with the currently used modem is that the forward power control bit validity is coupled with finger IN_LOCK and OUT_LOCK thresholds. The default values for IN_LOCK and OUT_LOCK thresholds are, for example, −32 dB and −37 dB, respectively. These two thresholds are very low. The primary consideration for choosing such low thresholds is the finger tracking performance. When the received signal-to-noise ratio is just above the OUT_LOCK threshold, the error rate for decoding the forward power control bit can be as high as 35% to 50%. As a result, the forward link traffic channel gain may be misadjusted.
For example, suppose a mobile station is at the edge of a cell and asks the base transceiver station to increase its transmitted forward link traffic power. Due to the decoding error, instead of increasing its transmitted power, the base transceiver station may incorrectly decrease its transmitted power. When the mobile station cannot correctly decode its received forward link traffic channel for a specific number of (e.g., 12) consecutive frames, it shuts down the transmitter thereof. Therefore, the base transceiver station can no longer hear the mobile station such that all fingers of the base transceiver station associated with that mobile station go out of lock. As a result, the forward link traffic channel power will stay at its current value until the call is dropped. The base transceiver station will try to decode the mobile's reverse link channel for a limited time, for example, five seconds. If not successful, the call will be eventually dropped.
It is, thus, necessary for controlling the forward link transmission power of the cellular mobile station to correctly demodulate a power-up bit transmitted by the mobile station.